diff --git a/.changeset/harness-cli-handler-options.md b/.changeset/harness-cli-handler-options.md
new file mode 100644
index 0000000000..c2f77f3e99
--- /dev/null
+++ b/.changeset/harness-cli-handler-options.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/ai-harness-cli': patch
+---
+
+`--serve` passes `canAccess` and `mediaSecret` to the harness handler, which needs both.
diff --git a/packages/ai-harness-cli/src/serve.ts b/packages/ai-harness-cli/src/serve.ts
index bdc3e67f2a..ed8c574847 100644
--- a/packages/ai-harness-cli/src/serve.ts
+++ b/packages/ai-harness-cli/src/serve.ts
@@ -101,6 +101,10 @@ export async function serve(
     host: options.host,
     harness: options.harness,
     authorize: (request) => (isAllowed(request) ? principal : null),
+    // One principal holds the token, so it can use every thread.
+    canAccess: () => true,
+    // ponytail: the token signs media URLs, so they work as long as it does.
+    mediaSecret: options.token,
   })
   const mcp = await mcpServer(options)
   const route = (request: Request) => {